Breath gases are measured in hospitals for several reasons and in several departments. Typical measured gases are one or several of the following gases: Carbon dioxide (CO2), Oxygen (O2), Nitrous oxide (N2O) and anesthetic gases (halothane, sevoflurane, desflurane, isoflurane and enflurane). Gases are measured either using sidestream technique (non-diverting) or mainstream technique (diverting). In the sidestream technique a gas sample is transferred through a sample line to a sensor, which causes a delay between breath and measurement. On the other hand, in the mainstream technique the gas sample is analyzed at the sampling site.
Variety of measurement techniques exist for measuring concentration of different gases. Paramagnetic analyzing technique is one of the side stream measurement techniques suitable for oxygen measurement. The paramagnetic measurement principle employs a paramagnetic sensor suitable for oxygen measurement as the oxygen has paramagnetic properties.
There are different kinds of paramagnetic oxygen sensors described in the prior art. However, one of the paramagnetic sensors comprises a reference flow, which is normal room air consisting of predetermined percentage of oxygen, and measurable flow of which oxygen concentration is to be analyzed. Both flow rates are approximately the same and they have the same pressure. The reference flow and the measurable flow are mixed in an air gap of a magnet having a strong magnetic AC-field. The AC magnetic field causes the oxygen molecules to move in a frequency that is double the magnet control frequency. This movement causes pressure variation, which can be measured for example with sensitive microphone(s). Because microphones are sensitive devices, they also sense mechanical and pneumatics interferences, which should be minimized.
One of the methods suggested in the prior art to measure the gas signal, uses a microphone connected between the reference flow and the measurable flow. The microphone measures the gas signal differentially and is well positioned to compensate the pneumatic interferences in normal situations but is sensitive to mechanical interferences.
Another method suggested in the prior art uses two microphones, one microphone for measuring the gas signal from the reference flow and another microphone for measuring the gas signal from the measurable flow. The two microphones used herein are connected using a single-end. The signals obtained from these two microphones are subtracted using suitable electronics.
The mechanical and the pneumatics interferences can be subtracted when the microphones are connected in the way described in the latter method. This helps in eliminating the mechanical and pneumatics interferences as the two microphones sense the pneumatics and mechanical interferences in the same way. However, in practice the mechanical and pneumatics interferences can't be eliminated altogether as there exists some sort of differences such as phase difference between the signals, as the signals pass through the microphones and their associated circuitry. Another limitation associated with the above-described methods is the difficulty associated with separating the gas signal from the mechanical and pneumatic interferences.
Hence, there exists a need to provide an accurate and reliable method for measuring oxygen concentration using a paramagnetic gas sensor.